Efficient Power Recovery in LNG LNG ... · PDF fileLNG Regasification Process ... working fluid, and requires also a heat sink. ... After ppgassing through the heat exchanger with

Efficient Power Efficient Power Recovery Recovery Efficient Power Efficient Power Recovery Recovery inin

LNG LNG RegasificationRegasification PlantsPlants

AuthorsAuthorsYoshitsugi Kikkawa Senior Engineering Consultant

Masaaki OhishiUnit Manager MEC Di Technolog & Engineering Senior Engineering Consultant

[email protected] Corporation

Yokohama, Japan

Unit Manager MEC Div. Technology & [email protected]

Chiyoda Corporation Yokohama, Japan

Chen-Hwa ChiuSenior Technology Advisor

chen hwa chiu@chevron com

Hans E Kimmel

[email protected] Energy Technology Company

Houston, Texas, USA

Michael Cords Hans E. KimmelExecutive Director R&[email protected]

Ebara International Corporation

Michael CordsSenior Mechanical Engineer

[email protected] International Corporation Ebara International Corporation

Sparks, Nevada, USA

pSparks, Nevada, USA

LNG Regasification ProcessLNG Regasification Process

Liquid Natural Gas is unloaded from LNG vessels ath i i i l d d i i l d kthe receiving terminal and stored in insulated tanks

at atmospheric pressure and a temperature of 111°Kelvin.

For regasification and distribution the LNG ispumped to high pressure and then heated topumped to high pressure and then heated tovaporize into its gaseous state.

Th h t t if th LNG i id d bThe heat to regasify the LNG is provided by seawater using the heat naturally stored in the sea orby other heat sources.

Power Recovery

LNG regasification plants are large heat sinks andrequire also large heat sources.

The differences in temperatures between the heatsources and the heat sinks are in the range of 170°sources and the heat sinks are in the range of 170Celsius providing the preconditions for an efficientpower recovery.

The Rankine Cycle is a thermodynamic cycle whichconverts heat into work. The heat is suppliedexternally to a closed loop with a particularworking fluid, and requires also a heat sink. Thiscycle generates about 80% of all global electricy g 80% gpower.

Power Recovery Using a ki C lRankine Power Cycle

The ideal Rankine Cycle consists of the following four processes:

1→2 Isentropic compression to highpressure in a pump

2→3 Constant high pressure heat additionin a boiler

3→4 Isentropic expansion in a turbine3 4 Isentropic expansion in a turbineexpander to low pressure

4→1 Constant low pressure heat rejection in a condensera condenser

Ideal Rankine Power Cycle

Rankine Cycle with High VapourContent Two-Phase Expansion

Rankine Cycle with Low Vapoury pContent Two-Phase Expansion

Thermodynamic Efficiency f h R ki P C lof the Rankine Power Cycle

Specific work input to pump: win = h2 – h1Specific work output from expander: wout = h3 – h4Specific heat input from 2 to 3: q h hSpecific heat input from 2 to 3: qin = h3 – h2

Net power output: wnet = wout – win

The thermodynamic efficiency of the ideal cycle is the ratio of net power output to heat input. at o o et po e output to eat put

ηtherm = wnet /qin

ηtherm = 1 – (h4 – h1)/(h3 – h2)

Schematic of Rankine Cycle Schematic of Rankine Cycle with Two-Phase Expansion

Schematic for Power Recovery Using a Cryogenic Working Fluid with a

Pump Two-Phase Expander Generator

For the power recovery in LNG re-gasificationp y gplants the proposed cryogenic working fluid for theRankine cycle is liquefied propane gas.

To achieve a higher efficiency the liquefied propanegas is passed through two heat exchangers and

t f t h d tone set of a pump two-phase expander generator,a compact assembly of a pump, a two-phaseexpander and an induction generator integrallymounted on one rotating shaft.

Schematic Description with Conventional Schematic Description with Conventional Cycle

1→2With work input, the pump pressurizes the liquid single phase workingfluid from low pressure to high pressure

2→32→3The pressurized working fluid is heated by passing through thegenerator and the heat exchanger with the heat provided by sea water orother heat sources

3→4The pressurized and heated working fluid expands from high pressure tolow pressure across the two-phase expander generating a work output

4→14→1The low pressure working fluid passes through a heat exchanger with theheat sink, the LNG for re-gasification. The working fluid condenses fromliquid-vapor two-phase to liquid single phase.

The compact assembly of a Pump Two-Phase Expander Generator

consists ofconsists of a pump,

a two-phase expander, and i d ti tan induction generator

integrally mounted on one rotating shaft.

Pump Two-Phase Expander p

Generator

Working fluid enters the pump at thelower inlet nozzle, exits the pump tothe side and passes through thethe side and passes through thegenerator housing cooling thegenerator, thus recovering the heatl f hlosses of the generator.

After passing through the heatp g gexchanger with the heat source, theworking fluid expands across thetwo-phase expander generating worktwo-phase expander generating workdriving the pump and the inductiongenerator

Pump Two Phase Expander Two-Phase Expander

Generator

In this modified design, thepressurized fluid passes directly fromthe pump through the generatorhousing cooling the generator, thenexit to the side and passing throughexit to the side and passing throughthe heat exchanger.

In both designs the leakage throughIn both designs, the leakage throughthe seal and the thrust is minimizeddue to equal pressure on both sidesof the seal and opposing directionsof the thrust forces.

Advantages of the Compact Assemblyg p y♦

The expander work output is larger than the pumpffwork input and the difference in work is converted by

the generator into electrical energy♦♦

The losses of a separate pump motor are eliminated♦♦♦

The losses of the induction generator are recovered andThe losses of the induction generator are recovered andused as heat source to heat the working fluid inaddition to the heat from sea water and other heatsources

♦♦♦♦Any leakage of the working fluid is within a closed loopy g g pand occurs only between pump and expander

Advantages of the Compact Assembly

♦♦♦♦♦Any leakage of the working fluid is minimized due toy g gequal pressure on both sides of the seal, and smallleakages are within a closed loop and occur onlybetween pump expander and generatorbetween pump, expander and generator.

♦♦♦♦♦♦The axial thrust is minimized due to opposingdi i f h h f d i h b idirections of the thrust forces decreasing the bearingfriction and increasing the bearing life.

♦♦♦♦♦♦♦The small physical size makes the unit ideal forfloating applications…

FSRUthe Floating Storage Regasification Unitsthe Floating Storage Regasification Units,

FPSOthe Floating Production Storage and Offloading Unitsthe Floating Production, Storage and Offloading Units,

are floating vessels used by the offshore industryare floating vessels used by the offshore industry for the processing, storage and transportation of LNG and for offloading the cargo in gaseous f t th d ti tiform at the destination.

Keppel Offshore & Marine Ltd © Keppel Offshore & Marine Ltd

FSRU “Golar Winter” Fl i S d R ifi i U i f G l LNG

© Keppel Offshore & Marine Ltd

Floating Storage and Regasification Unit for Golar LNG, Norway

• Even though the offshore regasification process issimilar to the onshore process, the design of anff h l h i ifi diffoffshore plant shows significant differences.

• Every square meter of an offshore footprint isy q prelatively expensive since it requires the support ofan offshore structure. The design has to be compactto keep the surface area as small as possibleto keep the surface area as small as possible.

• Due to the limited space, additional riskiti ti d HAZOP tmitigation measures and HAZOP assessments are

required.

• The continuous motion of the vessel impacts thedesign of the process equipment to be able to operateunder these dynamic conditions.

• Rotating equipment has to be designed toRotating equipment has to be designed towithstand the additional gyroscopic forces caused bythe vessel movements.

• The required design of any equipment is such thatthe center of gravity is as low as possible to increasethe stability of the vessel.

The Pump Two-Phase Expander Generator consists ofconsists of a pump,

a two-phase expander, and an induction generatoran induction generator

integrally mounted on one rotating shaft.

Each component is derived from existing field-proven equipment.field proven equipment.

Cryogenic high-pressure LNG pumps pressurize theLNG pumps pressurize the fluid up to the high pipe line pressure while it is still in the liquid state.

P G l D i C i iPump General Design Criteria

Liquid LNGModel 6ECC-1212Pump Design Pressure [bara] 133.4Lowest Design Temperature [°C] -168Operating Temperature [°C] -147Rated Flow [m³/hr] 287Rated Differential Head [m] 2396Design Specific Gravity .417Maximum Specific Gravity .451

Typical dimensions fordimensions for high-pressure LNG pumps:

4 meters in heightg

1 meter in diameter

12 pump stages with 300 mm

i ll diimpeller diameter

Particular design features for high-pressure LNG pumps: g p p p

Single piece rotating shaft with integrally mountedwith integrally mounted

multi-stage pump hydraulics and electrical induction motor

Thrust balancing mechanism to eliminate high axial thrust forces g

on the bearings

Electrical induction motor isElectrical induction motor is submerged in and cooled by LNG

B ll b i l b i dBall bearings are lubricated and cooled by LNG

Existing Field Proven Two-Phase Expanders

Cross section of a Two-Phase Liquefied Gas Expander inside pressurized containmentpressurized containment vessel with lower inlet and upper outlet nozzle

Rankine power cycle3→4 Isentropic two-phase expansion to a lower pressure

Two-Phasehydraulichydraulic assembly with nozzle ring (red),turbine runnerturbine runner (yellow), jet exducer(green) and two-phase draft tubedraft tube (metallic)

3 h l3→4 Isentropic two-phase expansion to a lower pressure

Nozzle Ring with converging nozzles generates high elocit orte flogenerates high-velocity vortex flow

Reaction turbine runner converts angular fluid momentum into shaft torqueangular fluid momentum into shaft torque

Jet Exducer

A radial outflow turbine for power generation byA radial outflow turbine for power generation byisentropic two-phase expansion to lower pressure

T Ph Two-Phase Expander Draft Tube

ffor pressure recovery

Power Generation with Ideal LiquidsPower Generation with Ideal LiquidsThe generated theoretical maximum mechanicalspecific power per mass P of expanders driven byspecific power per mass Pmax of expanders driven byideal liquids is equal to the product of specificvolumetric flow per second vs and the pressurediff Δ b d i l d ldifference Δp between expander inlet and outlet.

Pmax [J/(kg s] = vs[m3/(kg s)] Δp[Pa]max [J/( g ] s[ /( g )] p[ ]

33 Thermo-Fluid Dynamics of Two-phase Expanders

Power Generation with Real FluidsPower Generation with Real FluidsFor expanders driven by real fluids, likecompressible liquids gases and liquid vapourcompressible liquids, gases, and liquid-vapourmixtures, the specific volume v in m3/kg is notconstant and changes with the momentary pressurep and the enthalpy h.

v = v[h,p]v v[h,p]

34

The theoretical maximum differential enthalpy dh forll diff i l i d ia small differential expansion pressure dp is

described by the following differential equation

dh = v [h,p] dp

The generated theoretical maximum specific power isThe generated theoretical maximum specific power isthen calculated by integrating this differentialequation for Δh = h[p].Th di t t i kJ/ i Δh iThe corresponding power output in kJ/s is Δh inkJ/kg multiplied by the mass flow in kg/s.

Thermo-Fluid Dynamics of Two-phase Expanders

Field Experience with Liquefied Gas Two Phase Liquefied Gas Two-Phase

Expanders

Two-phase rich liquid feed p qexpanders installed in 2003 are operating successfully at PGNiG Odalanów PolandPGNiG, Odalanów, Poland.

Additional two-phase d i ll d i hexpanders are installed in the

feed to the lower column during 2009.g

36

Two-Phase Liquefied Gas Liquefied Gas Expanderat Ebara at Ebara Manufacturing

The presented Rankine Power Cycle, incorporating

a compact design consisting of a pump, a two-phase liquefied gasconsisting of a pump, a two phase liquefied gas expander and an induction generator, integrally

mounted on one single rotating shaft, offers an efficientoffers an efficient

and economical power recoveryfor LNG re-gasification plants

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